Recently, a molecular electronic device has been constructed [J. Chen, M. A. Reed, A. M. Rawlett, and J. M. Tour, Science 286, 1550 (1990)] that shows a negative differential resistance. We here present a simple theoretical interpretation of this experiment in terms of molecular orbital theory. We explain the negative differential resistance and, on the basis of our model, we suggest new experiments. The proposed mechanism turns out to be a modification of the standard mechanism of a solid state resonant tunneling diode.